Description

Limitation of power semiconductor devices • Majority carrier devices, like Schottky diode, MOSFET exhibit very fast switching responses, controlled essentially by the charging of the device capacitances. – However, forward voltage drops of these devices increases quickly with increasing breakdown voltage. • Minority carrier devices, like BJT, IGBT can exhibit high breakdown voltages with relatively low forward voltage drop. – But they can have longer switching times due to stored minority charges. • Energy is lost during switching transitions, due to a variety of mechanisms. – The resulting average power loss, or switching loss, is equal to this energy loss multiplied by the switching frequency. • So need of a mechanism to have a compensation between these issues.

THYRISTOR • Thyristor, a three terminal, four layers solid state semiconductor device, each layer consisting of alternately Ntype or P-type material, i.e; P-N-P-N, that can handle high currents and high voltages, with better switching speed and improved breakdown voltage . • Name ‘thyristor’, is derived by a combination of the capital letters from THYRatron and transISTOR. • Thyristor has characteristics similar to a thyratron tube which is a type of gas filled tube used as a high energy electrical switch and controlled rectifier. • But from the construction view point, a thyristor (pnpn device) belongs to transistor (pnp or npn device) family. • This means that thyristor is a solid state device like a transistor and has characteristics similar to that of a thyratron tube.

THYRISTORS • Thyristor (famous as Silicon Control Rectifier-SCR) can handle high currents and high voltages. • Typical rating are 1.5kA & 10kV which responds to 15MW power handling capacity. • This power can be controlled by a gate current of about 1A only. • Thyristor a three terminal (Anode, Cathode and Gate), three junctions and four layers solid-state semiconductor device, with silicon doped alternate material with P-N-P-N structure. • Thyristor act as bistable switches. – It conducts when gate receives a current pulse, and continue to conduct as long as forward biased (till device voltage is not reversed). – They stay ON once they are triggered, and will go OFF only if current is too low or when triggered off.

Thyristor- Operation Principle • Thyristor has three p-n junctions (J1, J2, J3 from the anode). • When anode is at a positive potential (VAK) w.r.t cathode with no voltage applied at the gate, junctions J1 & J3 are forward biased, while junction J2 is reverse biased. – As J2 is reverse biased, no conduction takes place, so thyristor is in forward blocking state (OFF state). – Now if VAK (forward voltage) is increased w.r.t cathode, forward leakage current will flow through the device. – When this forward voltage reaches a value of breakdown voltage (VBO) of the thyristor, forward leakage current will reach saturation and reverse biased junction (J2) will have avalanche breakdown and thyristor starts conducting (ON state), known as forward conducting state . • If Cathode is made more positive w.r.t anode, Junction J1 & J3 will be reverse biased and junction J2 will be forward biased. • A small reverse leakage current flows, this state is known as reverse blocking state. • As cathode is made more and more positive, stage is reached when both junctions A & C will be breakdown, this voltage is referd as reverse breakdown voltage (OFF state), and device is in reverse blocking state

Characteristics of Thyristors

Thyristor Operating modes Thyristors have three modes : Forward blocking mode: Anode is positive w.r.t cathode, but the anode voltage is less than the break over voltage (VBO) . • only leakage current flows, so thyristor is not conducting . Forward conducting mode: When anode voltage becomes greater than VBO, thyristor switches from forward blocking to forward conduction state, a large forward current flows. • If the IG=IG1, thyristor can be turned ON even when anode voltage is less than VBO. – The current must be more than the latching current (IL). – If the current reduced less than the holding current (IH), thyristor switches back to forward blocking state. Reverse blocking mode: When cathode is more positive than anode , small reverse leakage current flows. • However if cathode voltage is increased to reverse breakdown voltage , Avalanche breakdown occurs and large current flows.

Thyristor turn-ON methods • • Thyristor turning ON is also known as Triggering. With anode positive with respect to cathode, a thyristor can be turned ON by any one of the following techniques : – Forward voltage triggering – Gate triggering – dv/dt triggering – Temperature triggering – Light triggering

Forward Voltage Triggering • When breakover voltage (VBO) across a thyristor is exceeded than the rated maximum voltage of the device, thyristor turns ON. • At the breakover voltage the value of the thyristor anode current is called the latching current (IL) . • Breakover voltage triggering is not normally used as a triggering method, and most circuit designs attempt to avoid its occurrence. • When a thyristor is triggered by exceeding VBO, the fall time of the forward voltage is quite low (about 1/20th of the time taken when the thyristor is gate-triggered). • However, a thyristor switches faster with VBO turn-ON than with gate turn-ON, so permitted di/dt for breakover voltage turn-on is lower.

• • • • • • • • • Gate Triggering Turning ON of thyristors by gate triggering is simple and efficient method of firing the forward biased SCRs. In Gate Triggering, thyristor with forward breakover voltage (VBO), higher than the normal working voltage is chosen. – This means that thyristor will remain in forward blocking state with normal working voltage across anode and cathode with gate open. Whenever thyristor’s turn-ON is required, a positive gate voltage b/w gate and cathode is applied. With gate current established, charges are injected into the inner p layer and voltage at which forward breakover occurs is reduced. Forward voltage at which device switches to on-state depends upon the magnitude of gate current. – Higher the gate current, lower is the forward breakover voltage . When positive gate current is applied, gate P layer is flooded with electrons from cathode, as cathode N layer is heavily doped as compared to gate P layer. As the thyristor is forward biased, some of these electrons reach junction J2. As a result, width of depletion layer around junction J2 is reduced. – This causes junction J2 to breakdown at an applied voltage lower than forward breakover voltage VB0. If magnitude of gate current is increased, more electrons will reach junction J2, thus thyristor will get turned ON at a much lower forward applied voltage.

R Triggering v O a b LO A D i R R vS= V m 1 2 s in ωt V D R V g T

RC Triggering v O LO A D + R v S= V m D s in ω t V C C V 2 D 1 T

dv/dt triggering • With forward voltage across anode & cathode of a thyristor, two outer junctions (A & C) are forward biased but the inner junction (J2) is reverse biased. • The reversed biased junction J2 behaves like a capacitor because of the space-charge present there. • As p-n junction has capacitance, so larger the junction area the larger the capacitance. • If a voltage ramp is applied across the anode-to-cathode, a current will flow in the device to charge the device capacitance according to the relation: • If the charging current becomes large enough, density of moving current carriers in the device induces switch-on. • This method of triggering is not desirable because high charging current (Ic) may damage the thyristor.

Temperature Triggering • During forward blocking, most of the applied voltage appears across reverse biased junction J2. • This voltage across junction J2 associated with leakage current may raise the temperature of this junction. • With increase in temperature, leakage current through junction J2 further increases. • This cumulative process may turn on the SCR at some high temperature. • High temperature triggering may cause Thermal runaway and is generally avoided.

Light Triggering • In this method light particles (photons) are made to strike the reverse biased junction, which causes an increase in the number of electron hole pairs and triggering of the thyristor. • For light-triggered SCRs, a slot (niche) is made in the inner p-layer. • When it is irradiated, free charge carriers are generated just like when gate signal is applied b/w gate and cathode. • Pulse light of appropriate wavelength is guided by optical fibers for irradiation. • If the intensity of this light thrown on the recess exceeds a certain value, forward-biased SCR is turned on. Such a thyristor is known as light-activated SCR (LASCR). • Light-triggered thyristors is mostly used in highvoltage direct current (HVDC) transmission systems.

Thyristor Gate Control Methods • An easy method to switch ON a SCR into conduction is to apply a proper positive signal to the gate. • This signal should be applied when the thyristor is forward biased and should be removed after the device has been switched ON. • Thyristor turn ON time should be in range of 1-4 micro seconds, while turn-OFF time must be between 8-50 micro seconds. • Thyristor gate signal can be of three varieties. – D.C Gate signal – A.c Gate Signal – Pulse

Thyristor Gate Control Methods D.C Gate signal: Application of a d.c gate signal causes the flow of gate current which triggers the SCR. – Disadvantage is that the gate signal has to be continuously applied, resulting in power loss. – Gate control circuit is also not isolated from the main power circuit. A.C Gate Signal: In this method a phase - shifted a.c voltage derived from the mains supplies the gate signal. – Instant of firing can be controlled by phase angle control of the gate signal. Pulse: Here the SCR is triggered by the application of a positive pulse of correct magnitude. – For Thyristors it is important to switched ON at proper instants in a certain sequence. – This can be done by train of the high frequency pulses at proper instants through a logic circuit. – A pulse transformer is used for circuit isolation. – Here, the gate looses are very low because the drive is discontinuous.

Thyristor Commutation • Commutation: Process of turning off a conducting thyristor – Current Commutation – Voltage Commutation • A thyristor can be turned ON by applying a positive voltage of about a volt or a current of a few tens of milliamps at the gate-cathode terminals. • But SCR cannot be turned OFF via the gate terminal. • It will turn-off only after the anode current is negated either naturally or using forced commutation techniques. • These methods of turn-off do not refer to those cases where the anode current is gradually reduced below Holding Current level manually or through a slow process. • Once the SCR is turned ON, it remains ON even after removal of the gate signal, as long as a minimum current, the Holding Current (IH), is maintained in the main or rectifier circuit.

Thyristor Turn-off Mechanism • In all practical cases, a negative current flows through the device. • This current returns to zero only after the reverse recovery time (trr) , when the SCR is said to have regained its reverse blocking capability. • The device can block a forward voltage only after a further tfr, the forward recovery time has elapsed. • Consequently, the SCR must continue to be reverse-biased for a minimum of tfr + trr = tq, the rated turn-off time of the device. • The external circuit must therefore reverse bias the SCR for a time toff > tq. • Subsequently, the reapplied forward biasing voltage must rise at a dv/dt < dv/dt (reapplied) rated. This dv/dt is less than the static counterpart.

• • • • • • • Forced Commutation Applied to d.c circuits. If a thyristor is used in a DC circuit, when first turned on, it will stay on until the current goes to zero. To turn off the thyristor it is possible to use a Forced commutation circuit. The circuit creates a reverse voltage over the thyristor (and a small reverse current) for a short time, but long enough to turn off the thyristor. A simple circuit consist of a precharged capacitor and a switch (e.g. another thyristor) parallel to the thyristor. When the switch is closed, the current is supplied by the capacitor for a short while. This cause a reversed voltage over the thyristor, and the thyristor is turned off. Commutation is achieved by reverse biasing thyristor or reducing the thysristor current below the holding current value. Commutating elements such as inductor, capacitors are used for commutation purpose. Force commutation is applied to choppers and inverters. Force Commutation methods – Class A- Resonant Load – Class B- Self commutation – Class C- Auxiliary commutation – Class D- Complimentary commutation – Class E- External pulse commutation

Thyristor Turn-Off: Forced- Commutated Thyristor Circuit

THYRISTOR SWITCHING CHARACTERISTICS Thyristor Turn−ON time for a resistive Load Thyristor Turn −OFF time for a resistive Load

THYRISTOR turn-ON & turn-OFF Characteristics

Thyristor protection circuits • Reliable operation of a thyristor demands that its specified ratings are not exceeded. • In practice, a thyristor may be subjected to overvoltages or overcurrents. During SCR turn-on, di/dt may be prohibitively large. • There may be false triggering of SCR by high value of dv/dt. • A spurious signal across gate-cathode terminals may lead to unwanted turn-on. • A thyristor must be protected against all such abnormal conditions for satisfactory and reliable operation of SCR circuit and the equipment. • SCRs are very delicate devices, their protection against abnormal operating conditions is, therefore, essential. • The object of this section is to discuss various techniques adopted for the protection of SCRs. – di/dt protection. – dv/dtprotection.

di/dt protection • When a thyristor is forward biased and is turned on by a gate pulse, conduction of anode current begins in the immediate neighbourhood of the gate-cathode junction. • Thereafter, the current spreads across the whole area of junction. • The thyristor design permits the spread of conduction to the whole junction area as rapidly as possible. • However, if the rate of rise of anode current, i.e. di/dt, is large as compared to the spread velocity of carriers, local hot spots will be formed near the gate connection on account of high current density. • This localized heating may destroy the thyristor. Therefore, the rate of rise of anode current at the time of turn-on must be kept below the specified limiting value. • The value of di/dt can be maintained below acceptable limit by using a small inductor, called di/dt inductor, in series with the anode circuit. Typical di/dt limit values of SCRs are 20-500 A/µ sec. • Local spot heating can also be avoided by ensuring that the conduction spreads to the whole area as rapidly as possible. • This can be achieved by applying a gate current nearer to (but never greater than) the maximum specified gate current.

di/dt Protection • • A thyristor requires a minimum time to spread the current conduction uniformly throughout the junctions Otherwise, a localized “hot-spot” heating may occur due to high current density.

• • • • • • • • • • dv/dt protection With forward voltage across the anode & cathode of a thyristor, the two outer junctions (A & C) are forward biased but the inner junction (J2) is reverse biased. The reversed biased junction J2 behaves like a capacitor because of the space-charge present there. Let the capacitance of this junction be Cj. For any capacitor, i = C dv/dt. In case it is assumed that entire forward voltage va appears across reverse biased junction J2 then charging current across the junction is given by – i = dQ/dt =d(Cj Va )/dt – i=Cj (d Va /dt) + Va(d Cj /dt) – i = Cj dva /dt This charging or displacement current across junction J2 is collector currents of Q2 and Q1 Currents IC2, IC1 will induce emitter current in Q2, Q1. In case rate of rise of anode voltage is large, the emitter currents will be large and as a result, α1+ α2 will approach unity leading to eventual switching action of the thyristor. If the rate of rise of forward voltage dVa/dt is high, the charging current i will be more. This charging current plays the role of gate current and turns on the SCR even when gate signal is zero. Such phenomena of turning-on a thyristor, called dv/dt turn-on must be avoided as it leads to false operation of the thyristor circuit. For controllable operation of the thyristor, the rate of rise of forward anode to cathode voltage dVa/dt must be kept below the specified rated limit. Typical values of dv/dt are 20 – 500 V/µsec. False turn-on of a thyristor by large dv/dt can be prevented by using a snubber circuit in parallel with the device.

• • • • • Snubber circuit A snubber circuit consists of a series combination of resistance Rs and capacitance Cs in parallel with the thyristor as shown in Fig. Strictly speaking, a capacitor Cs in parallel with the device is sufficient to prevent unwanted dv/dt triggering of the SCR. When switch S is closed, a sudden voltage appears across the circuit. Capacitor Cs behaves like a short circuit, therefore voltage across SCR is zero. With the passage of time, voltage across Cs builds up at a slow rate such that dv/dt across Cs and therefore across SCR is less than the specified maximum dv/dt rating of the device. Here the question arises that if Cs is enough to prevent accidental turn-on of the device by dv/dt, what is the need of putting Rs in series with Cs ? The answer to this is as under.

snubber circuit (continue……) • Before SCR is fired by gate pulse, Cs charges to full voltage Vs. When the SCR is turned on, capacitor discharges through the SCR and sends a current equal to Vs / (resistance of local path formed by Cs and SCR). • As this resistance is quite low, the turn-on di/dt will tend to be excessive and as a result, SCR may be destroyed. In order to limit the magnitude of discharge current, a resistance Rs is inserted in series with Cs as shown in Fig. • Now when SCR is turned on, initial discharge current Vs/Rs is relatively small and turn-on di/dt is reduced. • In actual practice ; Rs, Cs and the load circuit parameters should be such that dv/dt across Cs during its charging is less than the specified dv/dt rating of the SCR and discharge current at the turn-on of SCR is within reasonable limits. • Normally, Rs Cs and load circuit parameters form an underdamped circuit so that dv/dt is limited to acceptable values.

Silicon-Controlled Rectifier (SCR) • • • SCR is a synonym of thyristor SCR is a four-layer pnpn device. – Has 3 terminals: anode, cathode, and gate. – In off state, it has a very high resistance. – In on state, there is a small on (forward) resistance. Applications: motor controls, time-delay circuits, heater controls, phase controls, etc.

Turning the SCR –ON Method and its Characteristics • • The positive pulse of current at the gate turns on Q2 providing a path for IB1. Q1 then turns on providing more base current for Q2 even after the trigger is removed. – Thus, the device stays on (latches).    The SCR can be turned on by exceeding the forward breakover voltage or by gate current. Notice that the gate current controls the amount of forward breakover voltage required for turning it on. VBR(F) decreases as IG is increased.

Turning SCR Off • • The SCR will conduct as long as forward current exceeds IH. There are two ways to drop the SCR out of conduction: – Anode Current Interruption – Forced Commutation.

Turning SCR Off : Anode Current Interruption • Anode current can be interrupted by breaking the anode current path , providing a path around the SCR, or dropping the anode voltage to the point that IA < IH.

Turning The SCR Off: Force Commutation • • Force commutation uses an external circuit to momentarily force current in the opposite direction to forward conduction. SCRs are commonly used in ac circuits, which forces the SCR out of conduction when the ac reverses.

SCR Applications - dc motor control • • • • SCRs are used in a variety of power control applications. One of the most common applications is to use it in ac circuits to control a dc motor or appliance because the SCR can both rectify and control. The SCR is triggered on the positive cycle and turns off on the negative cycle. A circuit like this is useful for speed control for fans or power tools and other related applications.

SCR Applications- crowbar circuits • • • • Another application for SCRs is in crowbar circuits (which get their name from the idea of putting a crowbar across a voltage source and shorting it out!) The purpose of a crowbar circuit is to shut down a power supply in case of over-voltage. Once triggered, the SCR latches on. The SCR can handle a large current, which causes the fuse (or circuit breaker) to open.

DIAC (diode for alternating current) • • • • • • • • • The DIAC is a five-layer device trigger diode that conducts current only after its breakdown voltage has been exceeded momentarily. When this occurs, the resistance of the diode abruptly decreases, leading to a sharp decrease in the voltage drop across the diode and, usually, a sharp increase in current flow through the diode. The diode remains "in conduction" until the current flow through it drops below a value characteristic for the device, called the holding current. Below this value, the diode switches back to its high-resistance (nonconducting) state. This behavior is bidirectional, meaning typically the same for both directions of current flow . – their terminals are not labeled as anode and cathode but as A1 and A2 or MT1 ("Main Terminal") and MT2. Most DIACs have a breakdown voltage around 30 V. DIACs have no gate electrode, unlike some other thyristors they are commonly used to trigger, such as TRIACs. diac is normally used in ac circuits The drawback of the diac is that it cannot be triggered at just any point in the ac power cycle; it triggers at its preset breakover voltage only. If we could add a gate to the diac, we could have variable control of the trigger point, and therefore a greater degree of control over just how much power will be applied to the line-powered device.

DIAC (diode for alternating current)

TRIAC (Triode for Alternating Current) • • • • • Triac is five layer device that is able to pass current bidirectionally and is therefore behaves as an a.c. power control device. In triac , the main connections are simply named main terminal 1 (MT1) and main terminal 2 (MT2). The gate designation still applies, and is still used as it was with the SCR. The useful feature of the triac is that it not only carries current in either direction, but the gate trigger pulse can be either polarity regardless of the polarity of the main applied voltage. The gate can inject either free electrons or holes into the body of the triac to trigger conduction either way. – • • • So triac is referred to as a "four-quadrant" device. Triac is used in an ac environment, so it will always turn off when the applied voltage reaches zero at the end of the current half-cycle. If we apply a turn-on pulse at some controllable point after the start of each half cycle, we can directly control what percentage of that half-cycle gets applied to the load, which is typically connected in series with MT2. This makes the triac an ideal candidate for light dimmer controls and motor speed controls. This is a common application for triacs.

Triac operation • • • • The triac can be considered as two thyristors connected in antiparallel as shown in Fig . The single gate terminal is common to both thyristors. The main terminals MT1 and MT2 are connected to both p and n regions of the device and the current path through the layers of the device depends upon the polarity of the applied voltage between the main terminals. The device polarity is usually described with reference to MT1, where the term MT2+ denotes that terminal MT2 is positive with respect to terminal MT1.

The Gate Turn-Off Thyristor (GTO)

GTOs Schematic representation

GTO Turn-on and Turn-off Pulses

Thyristor Summary • • • A thyristor is a latching device and it can be turned on with a small gate pulse, typically 100μs . Thyristors are generally off by line commutation due to the natural behavior of the input ac line supply. During the turn-off process, thyristors must be subjected to a reverse voltage for a certain minimum time known the turn-off.

Thyristor Conduction ia + vs _ ig v + vo _ s ωt vo ωt ig α ωt • Thyristor cannot be turned off by applying negative gate current. It can only be turned off if Ia goes negative (reverse) – This happens when negative portion of the of sine-wave occurs (natural commutation). • Another method of turning off is known as “forced commutation”, – The anode current is “diverted” to another circuitry.

Types of thyristors • Phase controlled – rectifying line frequency voltage and current for ac and dc motor drives – large voltage (up to 7kV) and current (up to 4kA) capability – low on-state voltage drop (1.5 to 3V) • Inverter grade – used in inverter and chopper – Quite fast. Can be turned-on using “force-commutation” method. • Light activated – Similar to phase controlled, but triggered by pulse of light. – Normally very high power ratings • TRIAC – Dual polarity thyristors

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